The invention relates to a method of manufacturing a rotary anode for X-ray tubes, in which a support member is manufactured from a molybdenum alloy and a target layer of tungsten or a tungsten alloy is provided on the support member by plasma spraying.
The invention also relates to the rotary anode obtained by means of said method.
It is an object ot the invention to provide a rotary anode which can be used in X-ray tubes which are used at a high load, for example, X-ray tubes for medical purposes.
German Patent Application No. 23 46 925 discloses a method of manufacturing an anode in which a target layer (i.e. the layer which is bombarded by the electrons when the rotary anode is used in an X-ray tube) of tungsten or a tungsten-rhenium alloy is provided on a support member of cast molybdenum or a molybdenum alloy. It is stated in the Patent Application that the target layer can be provided by plasma-spraying. However, details which might enable the production of dense layers by this process are not given.
With plasma-spraying of tungsten or tungsten alloys at atmospheric pressure, according to the literature, it is generally not possible to obtain a density higher than approximately 92-94% of the theoretical maximum density (see, for example, R. Glatzle et al, Metall 24, 823 et seq. 1970). Such a density is insufficient for rotary anodes; it is not possible to maintain a proper vacuum in the X-ray tube with such a density.
It has been tried to increase the density by densely sintering the tungsten layer. A maximum density of 97% of the theoretical maximum density is then obtained (R. Glatzle et al. ibid.). The prescribed sintering treatment (up to 15 hours at 2600.degree. C.) causes an unacceptable loss of strength of many molybdenum alloys normally used for support members so that the use in an X-ray tube becomes impossible. Plasma spraying tungsten under reduced pressure is known per se from Moses A. Levinstein, Cienca Y technica de la Soldadure (Madrid) 12, No. 66, pp. 1-9 (1962) (see also Chemical Abstracts 58, 1963), 4243f). However, a density of not more than 92.7% of the theoretical maximum density was then obtained. It is also stated that reduction of pressure resulted in lower densities.
A method of plasma spraying materials, for example, tantalum, tungsten carbide and the like, in which plasma currents are used at speeds of Mach 3 is known from E. Muehlberger "A high-energy plasma coating process", Proc. 7th Intern. Metal Spraying Conf. 1973, London (see also U.S. Pat. No. 3,839,618). To obtain such velocities, spraying is effected in a chamber at a pressure of less than half an atmosphere and preferably much less. However, experiments have demonstrated that although in this manner layers of sufficient ensities are obtained, the resulting layers nevertheless are unfit to serve as target layers because in the method described the necessary process conditions cause the tungsten particles virtually to be deposited at a temperature which is too low because a globular structure having too little mutual bonding between the particles is obtained.